1. Field of the Invention
The present invention relates to a head position control method for a disk device, head position control device and disk device, and more particularly to a head position control method, head position control device and disk device for suppressing position shift by disturbance, using observer control.
2. Description of the Related Art
In a disk device, such as a magnetic disk device and optical disk device, it is extremely important to accurately position the head on the target track to improve recording density.
In this positioning control, it is known that disturbance influences positioning accuracy. To suppress this disturbance by a control system, the control systems shown in FIG. 24 to FIG. 26 have been proposed. In the first prior art in FIG. 24, the position error “e” between the target position “r” and the current position “y” of the plant 108 is computed by the computing block 100, the result is input to the controller 102, and the controller 102 computes the control amount to decrease the position error “e”. The position control system is added the reverse characteristic filter 104 of the notch filter in parallel to the feed back control system for driving the plant 108. The filter 104 generates a disturbance suppress amount from the position error “e”, and suppresses a component around a specific frequency of the position error by adding to the control amount through a computing block 106 (see patent document: U.S. Pat. No. 6,487,028 B1).
In the second prior art, as shown in FIG. 25, the filter 104 is installed in series with the controller 102 of the feed back loop in FIG. 24, and a component around a specific frequency of the control amount of the controller 102 is suppressed (see non-patent document: R. J. Bickel and M. Tomizuka, “Disturbance observer based hybrid impedance control”, Proceedings of the American Control Conference, 1995, pp. 729 to 733).
In the third prior art, as shown in FIG. 26, called as the disturbance observer is added to the feed back loop. That is, the difference between the value when the current position “y” is divided by the transfer function P of the plant 108 in the block 110, that is the twice differential value of the position error, and the current value instructed from the computing block 106, is determined by the computing block 112. And the computed difference is fed back to the computing block 106 via the band pass filter (also called Q filter) 114 using the feed back loop (see above non patent document).
To handle the eccentricity of the disk, which is a periodic disturbance, a method of correcting eccentricity using an eccentricity estimation observer has been proposed (e.g. patent document: Japanese Patent Application Laid-Open No. H7-50075, or Japanese Patent Application Laid-Open No. 2000-21104).
The eccentricity estimation observer computes the control value of the actuator from the error between the actual position error and the estimated position error using the state estimation gains A, B, C, F and L, and computes the state quantity (position, velocity, bias value, eccentricity amount) of the next sample.
Here the estimation gain L is comprised of the position estimation gain L1, velocity estimation gain L2, bias estimation gain L3 and eccentricity estimation gains L4 and L5. And L1, L2 and L3 indicate the characteristics of the controller itself, and L4 and L5 indicate the response characteristic to eccentricity, which is a periodic disturbance.
Positioning control to follow up to external vibration other than the eccentricity component using such an observer is demanded. In other words, as the recording density of the disk device increases, the influence of external vibration on the positioning accuracy of the head can no longer be ignored. For example, the vibration of a medium, or wind which the head receives by the rotation of a medium, influences the positioning accuracy of the head. Also as the use of disk devices expands, disk devices are now mounted on mobile equipment, such as a portable terminal, portable telephone and portable AV equipment, and adaptation to a wide range of disturbance frequencies is also demanded.
In the case of disturbance suppression of the above mentioned prior arts, if a compensator for selectively suppressing a specific frequency range, such as eccentricity correction, is added, the characteristics of the original control system are not influenced if the width of the suppression range is set to be extremely narrow. However along with adapting to a wide range of disturbance frequencies that is now demanded, the characteristics of the original control are affected if the suppression width is set to be wide, or if the disturbance of a high frequency range is suppressed, therefore adding a desired disturbance suppression function is difficult.
Also in the case of the prior arts, if a disturbance suppression function is added after one observer is designed, the characteristics of the entire control system, such as pole positioning, shift considerably, and a redesign of the entire observer is required. In other words, in the case of the prior arts where the disturbance models are determined and then the observer is designed including the controller and disturbance suppression functions, if a specific disturbance suppression function is added after, the entire observer is influenced and redesign is required.